The functional state of endothelium can be described as three types of conditions: 1) Basal physiological condition where endothelium maintain a balance in vascular tone that could be adapted to maintain the blood flow according the tissue metabolic requirements also preventing thrombosis and regulating the normal fluid distribution between intravascular and interstitial space and allowing the innate immune system to protect the organism. 2) An Activated physiological state, where the endothelial cells became stimulated by different mediators or agents that allow it to acquire a new endothelial function which is going to be beneficial for the host, i.e. the defense response to an antigen or micro organism or to repair a tissue damage, among others, and 3) A Dysfunctional state, where the endothelium failure to adequately perform its homeostatic function thereby the principal alterations are the deregulated vascular tone (increased or decreased), usually associated to a predominantly protrombotic activity, which also can be associated to a deterioration in the control of liquid and molecules movement through the vascular wall. Significant injury damages the endothelial cell and the subsequent dysfunctional state usually contributes to a vicious circle where both the endothelium and the entire organism are progressively to be damaged. © 2012 by Nova Science Publishers, Inc. All rights reserved.